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Tulu Kapi |
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Ethiopia |
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Au
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Super Porphyry Cu and Au
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The Tulu Kapi gold project is located in the West Oromia region of Ethiopia, ~28 km east of the town of Ayra-Gulliso and ~550 km west of Addis Ababa (#Location: 9° 4' 55"N. 35° 33' 6"E).
The deposits are situated within the West Ethiopia Terrane in the southwestern section of the Arabian-Nubian Shield.
Gold and platinum have been historically mined in the Tulu Kapi district from alluvial and eluvial accumulations. Tulu Kapi mineralisation has been mined and 're-discovered' several times throughout the 20th and 21st centuries. Early in the 20th Century it was part of an East African gold rush (Zaccaria, 2005; Maiocchi, 2015). Subsequently, following the Italian invasion of Ethiopia in 1935, the Italian Colonial Mining Service was set up to carry our a rapid exploration program across Ethiopia resulting in, among other areas, exploitation of aluvial and alluvial gold in the district surrounding Tulu Kapi. In 1939, reserves of approximately 1.12 t of gold in alluvials, saprolite and in quartz veins was reported at Tulu Kapi (UNDP, 1980). The area of saprolite mining is located in the centre of the main resource in the currently outlined Tula Kapi deposit. Due to high costs, the Italian exploitation of Tulu Kapi ceased in 1941. Following its creation in 1968, the Geological Survey of Ethiopia undertook reconnaissance and detailed mapping mineral surveys with the United Nations Development Program (UNDP) from 1969 to 1972. This work outlined the 'Nejo-Yubdo' mineralised belt, which includes the Tulu Kapi deposit, the parallel satellite gold trends and many sites of historical and active alluvial, eluvial and hard rock gold mining. However, despite encouraging results, there was no significant exploration activity in the Tulu Kapi-Yubdo area between 1973 and 1996, due to periods of war, civil war and famine. In 1996, a Canadian company Tan-Range Resource drilled at Tulu Kapi and nearby but withdrew due to a drop in the gold price and renewal of the conflict with Eritrea. From 2004 to 2006, Golden Prospect Mining Company (GPMC), owned subsidiary of Minerva Resources, acquired a series of exploration licences in the district, based on earlier exploration results. That company drilled the 'discovery' hole in 2006 with an intersection of 37 m @ 4.61 g/t Au. GPMC was acquired by Nyota Minerals in 2009, and soon after, a maiden resource of 21.5 t of contained gold was announced. In 2013-14, Kefi Minerals tok over Nyota Minerals, and after further expansion of the deposit and optimisation of the mining plan and completed plans to begin a mining operation (Granitzio et al., 2017).
Regional Setting
For a description of the regional setting of the Shield and its geology and distribtion of mineralisation, see the separate Arabian Nubian Shield Overview record.
The rocks of the West Ethiopia Terrane are subdivided into older high-grade pre-Neoproterozoic gneisses, with interleaved slivers of supracrustal rocks, and low-grade Neoproterozoic volcano-sedimentary sequences and associated dismembered mafic-ultramafic bodies. The rocks are intruded by syn- to post-tectonic felsic to mafic plutons. Western Ethiopia is generally characterised by north-south to NNE-NNW, and minor but important NW-SW structural trends. The dismembered mafic-ultramafic complexes are interpreted to be remnants of Neoproterozoic suture related ophiolitic sequences.
Three main deformation events have been differentiated in the Tulu Kapi area of West Ethiopia Terrane:
D1 is represented by west-vergent thrust and related recumbent tight to isoclinal folds with sub-horizontal axis, and a shallow, SE dipping and NE-SW-trending axial planar penetrative foliation S1.
D2, which is as progressive continuation of D1 and is characterised by upright folds and sinistral NE oriented strike-slip shear zones, which resulted in the steepening of D1 structures.
D3, defined by conjugate brittle-ductile strike-slip faults/shear zones that are superimposed at high angles to the D1 and D2 structures. The principal structure of this event is the NW trending Didessa shear zone. Local open east-west folds are related to the continuation D3 deformation.
Three main NNE-SSW trending structural/mineralised trends are recognised ib the Tulu Kapi district. The easternmost is the the main Tulu Kapi trend, which includes the main deposit. The Guji Trend, is ~3.5 km to the west and includes a series of prospects distributed over an interval of >30 km. The third, the Dina Trend take in a number of prospects and historic workings over a strike length of ~15 km, ~9.5 km further to the west.
Geology and Mineralisation
The geology of the Tulu Kapi district comprises Neoproterozoic meta-sedimentary and meta-volcanic rocks that characteristically occupy deeply incised valleys which separate prominent hills of intrusive rocks.
Gold mineralisation occurs in quartz-albite alteration zones that occur as stacked sub-horizontal lenses in a syenite pluton into which a swarm of dolerite dykes and sills have been intruded. Mineralisation is distributed over a 1500 x 500 m zone with a vertical extent of >550 m below the surface, and is characterised by a mineralogy comprising free gold, silver and pyrite with minor sphalerite and galena. Gold recovery is estimated to be ~93% for both oxide and sulphide ore. These alteration lenses are interpreted to originate from the steeply NW dipping Bedele Shear which occurs along the southeastern margin of the deposit. This shear zone obliquely transects the steep boundary between a diorite intrusion to the SE and the mafic syenite that hosts mineralisation to the NW.
Beneath the main albitised lenses, within mineralised syenite, at depth, there is a steeply dipping zone adjacent to the Bedele Shear Zone that is characterised by significantly higher gold grades. This zone has occasional coarse visible gold, more base metal sulphides and a shallower apparent dip than the main body above.
The principal mineralisation stage produced a very variable assemblage of hydrothermal minerals that include quartz, albite, carbonate, biotite, muscovite, epidote, sulphide and gold, and commonly occurs in veins, crackle zones and minor breccia zones.
Within the pit outline gold mineralisation occurs as replacement and fracture fill quartz veins associated with sulphide-bearing albite alteration. Albite overgrows original feldspar within the syenite, as well as forming new crystals in veins and cavities. The syenite has undergone pervasive albitisation, which is cut by quartz veins that pre-date gold mineralisation. This supports an interpretation that the albite associated with the mineralisation is a later event. These are all overprinted by a late stage development of weak, <10 µmm gold free pyrite veins that crosscut muscovite veins.
Sulphide minerals on the ore zone include pyrite, sphalerite, bornite, chalcopyrite, galena and arsenopyrite, with possible tetrahedrite-tennantite Pollard (2008). Pyrite, which is by far the most abundant sulphide, occurs as coarse- to fine-grained vein fill and as alteration spots in syenite. Aggregates of small pyrite crystals are also common, typically containing crystals of silicate and carbonate minerals to form a sieve-like texture. The absence of appreciable arsenic, antimony and tellurium minerals suggest that the majority of gold is free gold, which has been shown to be predominantly non-refractory.
Ore Reserves and Mineral Resources
Probable Ore Reserves as at 22 April, 2015 (KEFI Minerals website viewed March, 2020):
High grade at 0.90 g/t Au cut-off - 12.0 Mt @ 2.52 g/t Au;
Low grade between 0.50 and 0.90 g/t Au cut-offs - 3.3 Mt @ 0.73 g/t Au;
TOTAL Ore Reserve - 15.4 Mt @ 2.12 g/t Au for 32.65 t of gold.
Mineral Resources as at 22 April, 2015 (KEFI Minerals website viewed March, 2020):
Indicated + Inferred Resource above RL 1400 m at a 0.45 g/t Au cut-off - 19.0 Mt @ 2.46 g/t Au;
Indicated + Inferred Resource below RL 1400 m at a 2.5 g/t Au cut-off - 1.2 Mt @ 5.69 g/t Au;
TOTAL Ore Reserve - 20.2 Mt @ 2.65 g/t Au for 53.5 t of gold.
NOTE: Mineral Resources are inclusive of Ore Reserves.
The most recent source geological information used to prepare this decription was dated: 2017.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Tulu Kapi
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Selected References:
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Bowden, S., Gani, N.D., Alemu,T., O Sullivan, P., Abebe, B. and Tadesse, K., 2020 - Evolution of the Western Ethiopian Shield revealed through U-Pb geochronology, petrogenesis, and geochemistry of syn- and post-tectonic intrusive rocks: in Precambrian Research v.338, 14p. doi.org/10.1016/j.precamres.2019.105588.
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Granitzio, F., Rayner, J. and Aregay, T., 2017 - Tulu Kapi Gold Project: A history of repeated discoveries in Western Ethiopia: in NewGenGold 2017, Conference, Case Histories of Discovery, 14-15 November 2017, Perth Western Australia, Paydirt Media, Perth, Conference Proceedings, pp. 85-101.
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Johnson, P.R., Zoheir, B.A., Ghebreab, W., Stern, R.J., Barrie, C.T. and Hamer, R.D., 2017 - Gold-bearing volcanogenic massive sulfides and orogenic-gold deposits in the Nubian Shield: in S. Afr. J. Geol. v.120, pp. 63-76.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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